There are many methods for manufacturing golf balls. One type of golf ball includes a tensioned material wound around a spherical center. Another type of golf ball uses solely solid layers, typically of thermoset or thermoplastic materials. Wound balls typically have either a solid rubber or fluid center around which many yards of a tensioned elastic thread, typically polyisoprene, are wrapped to form a wound core. One or more added layers of thermoset or thermoplastic materials may surround the thread layer to complete the golf ball construction. Prior art golf balls with liquid centers have traditionally been enclosed by a layer of wrapped elastic material.
A wound material layer differs from a solid resilient material layer in that the wound layer is often able to more readily elongate and compress in a direction lateral to the impacting force. For this reason, wound golf balls have a tendency to more easily compress at impact as compared to a solid golf ball (Dalton, Golf and Science III, 1999).
It has long been the goal of golf ball manufacturers to create a non-wound construction golf ball with the performance and feel properties of wound balls. Golf ball designs have been introduced which use multilayer non-wound constructions. These include double cover designs with solid, single member cores; dual core designs with two core members and a single cover layer; balls with multiple core and/or multiple cover layers; and balls with liquid centers that do not have elastic windings. U.S. Pat. Nos. 5,480,155 and 5,150,906 by Molitor et al., 5,683,312 and 5,919,100 by Boehm et al. are examples of non-wound liquid center balls. Hollow golf balls having a spherical cavity in the center are disclosed by Boehm et al. and further described in U.S. Pat. No. 5,944,621 to Tsujinaka et al.
One technique suggested in the prior art to avoid the problem of an overly hard stiff cover was disclosed in U.S. Pat. No. 4,431,193 issued to Nesbitt. Rather than have a single layer cover over the core, the cover would be molded in two layers: a hard stiff inner layer of a high flexural modulus material that provides significant hoop stress, surrounded by a soft, flexible outer cover of a lower flexural modulus material. Balls of this design have been sold bearing the Strata name for some time, however, because of the inner layer thickness of about 0.045 inches to 0.050 inches and the high flexural modulus of greater than 50,000 psi, the golf balls have a hard feel to which many golfers object.
U.S. Pat. No. 5,713,801 issued to Aoyama teaches a method for making a golf ball providing a core of solid resilient material, winding a high elastic modulus fiber on the core to create a first wound layer to form a “hoop-stress layer,” and molding an outer layer of resilient material about the first wound layer. The invention includes a golf ball having a substantially spherical core, a first wound layer of high tensile elastic modulus fibers wound about the core, and a second molded layer of a resilient material surrounding the wound layer The core in the above method and apparatus may also be made of a center wound with a low modulus fiber and provided with an initial tension.
U.S. Pat. No. 5,913,736 issued to Maehara et al. builds upon Aoyama to describe a hoop-stress layer made of a shape memory alloy (Ti—Ni) wound around a core so as to provide a shaped memory alloy layer.
Thus, it would be advantageous to provide a golf ball having at least four or more layers, including a hoop-stress layer and at least one resilient elastomeric layer, to form a golf ball with improved performance characteristics.